1. Field of the Invention
The present invention relates to a seat track having a lower rail provided on the floor side, an upper rail configured to movably engage the lower rail and allow a seat to be mounted thereon, a retainer arranged in a space extending in the fore-and-aft direction and the vertical direction between the lower rail and the upper rail, a lower ball and an upper ball rotatably supported respectively by an upper portion and a lower portion of the retainer and configured to come into contact with both the lower rail and the upper rail.
2. Description of the Related Art
Referring now to FIG. 8 and FIG. 9, a seat track will be described. FIG. 8 is an exploded perspective view of a seat track in the related art, and FIG. 9 is a cross-sectional view taken along the line IX-IX when the seat track in FIG. 8 is assembled.
In these drawings, a seat track 1 includes a pair of lower rails provided on the floor side, and upper rails 5, 5′ configured to movably engage the lower rails 3 and allow a seat to be mounted thereon.
Arranged between the lower rail 3 and the upper rails 5, 5′ are retainers 7. Two lower balls 9, 9′ and two upper balls 11, 11′ which come into contact with both the lower rail 3 and the upper rails 5, 5′ are rotatably supported by upper portions and a lower portions of the retainers 7 so as to reduce the sliding resistance between the lower rail 3 and the upper rails 5, 5′ (for example, see JP-A-2000-233670).
In the seat track configured as described above, the hardness of the lower balls 9, 9′ and the upper balls 11, 11′ is higher than that of the lower rail 3 and the upper rails 5, 5′. Therefore, when the upper rails 5, 5′ are used at the same position for a long time without being moved, indentations (indentations) are formed on the lower rail 3 and the upper rails 5, 5′ by the lower balls 9, 9′ and the upper balls 11, 11′ due to the weight of a person seated thereon or a tensile load of a belt anchor. In this specification, the position of the upper rail 5 where these indentations are formed is referred to as “initial position”.
Then, when the upper rails 5, 5′ are moved from the initial positions, the lower balls 9, 9′ and the upper balls 11, 11′ may fall into the indentations, so that vibrations in the vertical direction may occur on the upper rails 5, 5′.
Referring now to FIGS. 10A to 10G, the vibrations in the vertical direction of the upper rails caused by the balls and the indentations will be described. FIG. 10A to FIG. 10G are configuration drawings in cross section taken along the line X-X in FIG. 9. The center-to-center distance of the lower ball 9 and the lower ball 9′ of the retainers 7 is 10 mm, and the center-to-center distance of upper ball 11 and the upper ball 11′ is 10 mm. The diameters of the lower balls 9, 9′ and the upper balls 11, 11′ are set so that the distance of movement of the centers of the lower balls 9, 9′ and the upper balls 11, 11′ become half the distance of movement of the upper rails 5, 5′.
FIG. 10A shows a state in which the upper rails 5, 5′ are used at the same position for a long time without being moved, that is, the initial position. The lower rail 3 and the upper rail 5 are formed with the indentations at points indicated by “X” by the lower balls 9, 9′ and the upper balls 11, 11′.
An indentation formed by the lower ball 9 on the lower rail 3 is designated by LL1, and an indentation formed by the lower ball 9 on the upper rail 5 is designated by LU1. An indentation formed by the lower ball 9′ on the lower rail 3 is designated by LL2, and an indentation formed by the lower ball 9′ on the upper rail 5 is designated by LU2. An indentation formed by the upper ball 11 on the lower rail 3 is designated by UL1, and an indentation formed by the upper ball 11 on the upper rail 5 is designated by UU1. An indentation formed by the upper ball 11′ on the lower rail 3 is designated by UL2, and an indentation formed by the upper ball 11′ on the upper rail 5 is designated by UU2.
Here, when the upper rail 5 is moved in one direction (leftward in the drawing) by 10 mm from the initial position, as shown in FIG. 10B, the centers of the lower balls 9, 9′ and the upper balls 11, 11′ move by 5 mm in the one direction.
When the upper rail 5 is moved from the state shown in FIG. 10B by 10 mm (20 mm from the initial position) in the one direction, as shown in FIG. 10C, the lower ball 9 falls into the indentation LU2 on the upper rail 5, the lower ball 9′ falls into the indentation LL1 on the lower rail 3, the upper ball 11 falls into the indentation UU2 on the upper rail 5, and the upper ball 11′ falls into the indentation UL1 on the lower rail 3. In other words, all the balls fall into the indentations, and the vibrations in the vertical direction occur in the upper rail 5.
When the upper rail 5 is further moved in the one direction from the state shown in FIG. 10C, as shown in FIG. 10D, the lower balls 9, 9′ and the upper balls 11, 11′ are positioned between the indentations LU1, LU2, UU1, and UU2 on the moving upper rail 5 and the indentations LL1, LL2, UL1, and UL2 on the fixed lower rail 3, so that the lower balls 9, 9′ and the upper balls 11, 11′ do not fall into the indentations.
In the same manner, when the upper rail 5 is moved in the other direction (rightward in the drawing) by 5 mm from the initial position, as shown in FIG. 10E, the centers of the lower balls 9, 9′ and the upper balls 11, 11′ move by 5 mm in the other direction.
When the upper rail 5 is moved from the state shown in FIG. 10E by 10 mm (20 mm from the initial position) in the other direction, as shown in FIG. 10F, the lower ball 9 falls into the indentation LL2 on the lower rail 3, the lower ball 9′ falls into the indentation LU1 on the upper rail 5, the upper ball 11 falls into the indentation UL2 on the lower rail 3, and the upper ball 11′ falls into the indentation UU1 on the upper rail 5. In other words, all the balls fall into the indentations, and the vibrations in the vertical direction occur in the upper rail 5.
When the upper rail 5 is further moved in the other direction from the state shown in FIG. 10F, as shown in FIG. 10G, the lower balls 9, 9′ and the upper balls 11, 11′ are positioned between the indentations LU1, LU2, UU1, and UU2 on the moving upper rail 5 and the indentations LL1, LL2, UL1, and UL2 on the fixed lower rail 3, so that the lower balls 9, 9′ and the upper balls 11, 11′ do not fall into the indentations.
In other words, in the configuration as shown in FIGS. 10A to 10G, when the upper rail 5 is moved, all the balls, that is, the lower balls 9, 9′ and the upper balls 11, 11′ fall into the indentations at three points shown FIG. 10A (the initial position), FIG. 10C, and FIG. 10F, and vibrations in the vertical direction occur in the upper rail 5.